Lecture 4 • Eclipses • Geocentric vs Heliocentric Theory •The Nature of Scientific Theories The Planets Prof.
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Lecture 4 • Eclipses • Geocentric vs Heliocentric Theory •The Nature of Scientific Theories
The Planets
Prof. Geoff Marcy © 2005 Pearson Education Inc., publishing as Addison-Wesley
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Crescent Moon and Venus visible sunday night .
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Review of Last Lecture
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Motion of the Night Sky as hours pass
1. From Berkeley 2. From the Equator 3. From the North Pole © 2005 Pearson Education Inc., publishing as Addison-Wesley
Berkeley Hills © 2005 Pearson Education Inc., publishing as Addison-Wesley
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Moon: Going through phases each 29.5 Days (one orbital period) © 2005 Pearson Education Inc., publishing as Addison-Wesley
If you stand on the Moon, does the Earth go through “phases” ? Yes !
When Earthlings see a crescent moon, what is the phase of the Earth, as seen from the Moon?
a. New b. Crescent c. Quarter d. Gibbous © 2005 Pearson Education Inc., publishing as Addison-Wesley
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Section
1
Eclipses: Solar and Lunar
• What do they look like?
• Why do they happen?
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Eclipses
Solar Eclipse: Moon blocks the Sun
• Sun is behind the moon: Only occurs at new moon You are in the Moon’s shadow.
within umbra: total solar eclipse within penumbra: partial solar eclipse © 2005 Pearson Education Inc., publishing as Addison-Wesley
2002 total Solar Eclipse Ceduna, Australia Dec 4, 2002 © 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse
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Solar Eclipse 1999 Aug 11 from the Russian Mir Space Station © 2005 Pearson Education Inc., publishing as Addison-Wesley
Eclipses
• The Earth & Moon cast shadows.
• When either passes through the other’s shadow, we have an
eclipse
.
• Why don’t we have an eclipse every full & new Moon?
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Sun
Total Solar Eclipse Lusaka, Zambia 2001 © 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse in India 24 October 1995 By: Solar Physicsts Wendy Carlos and Fred Espenak India © 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse
May 20 2012 Just after Finals Last Spring Sproul Plaza
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Solar Eclipse Predictions
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Total Solar Eclipse: Aug 21, 2017 © 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse: Aug 21, 2017
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Lunar Eclipses
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Section
2 A Model of the Solar System: Geocentric vs Heliocentric © 2005 Pearson Education Inc., publishing as Addison-Wesley
We see only one side of the Moon © 2005 Pearson Education Inc., publishing as Addison-Wesley
Why we always see the same face of the Moon
Rotation period = orbital period Not Correct Model Correct Model Moon Moon Earth Earth © 2005 Pearson Education Inc., publishing as Addison-Wesley
Build a
model
, i.e. conceptual
theory
, of the Solar System
Conceptual models must explain all the motions of the planets: i.e., the “data”. . .
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Planets: Moving relative to the Stars
Obvious to the eye • Mercury – Difficult to see; Always angularly close to Sun • Venus – Very bright. Always near Sun— morning or evening “star” • Mars – Noticeably orange. Usually moves west-to-east – Sometimes backwards “retrograde” !
• Jupiter – Very bright. Moves west-to-east against stars.
• Saturn – Moderately bright. Moves more slowly west-to-east.
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Motion of the Planets
Relative to the Earth
during several years Earth The Sun and planets seem to orbit the earth during months and years .
Here: Sun Mercury Venus Mars © 2005 Pearson Education Inc., publishing as Addison-Wesley
The Motion of Planets • The Planets normally move West to East against the background stars.
• Why do planets sometimes seem to move backwards relative to the stars?
• Greeks concluded that the planets orbit the Earth. Why did smart people conclude this?
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Observed Motion of Mars: Normal & “Retrograde”
”
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To Explain Retrograde Motion
Two Models:
Geocentric Heliocentric © 2005 Pearson Education Inc., publishing as Addison-Wesley
Ptolemy’s
Geocentric
Model
• Earth is at center • Sun orbits Earth • Planets orbit on small circles whose centers orbit the Earth on larger circles (The small circles are called
epicycles
) © 2005 Pearson Education Inc., publishing as Addison-Wesley
In Heliocentric “model”
Retrograde Motion is a Natural Consequence • Planets usually appear to move eastward relative to the stars.
• But as we pass by them, planets seem to move west relative to the stars. • Only noticeable over many nights; on a single night, a planet rises in east and sets in west… © 2005 Pearson Education Inc., publishing as Addison-Wesley
Two Theories: : Earth-Centered Theory Sun-Centered Theory Which theory Seems “Best” ?
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Choosing the Best Model: Explaining Retrograde Motion • Natural result of Heliocentric Model • Difficult to explain if Earth is at center The Best “Model” or “Theory” explains various data and phenomena with the fewest assumptions.
“Occam’s Razor “: Choose the simplest model that explains all the data.
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3D “model” of the Solar System © 2005 Pearson Education Inc., publishing as Addison-Wesley
Why did the Greeks reject the theory that the Earth orbits the Sun?
• It ran contrary to their common sense: Every day, the sun, moon, and stars rotates around us. So, we “must be” at the center . . .
• If the Earth rotated, then there should be a “great wind” as we moved through the air.
• Greeks knew that we should see stellar “parallax” if we orbited the Sun – but they could not detect it. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Test the Theory that Earth orbits the Sun:
Parallax:
Apparent shift of a star’s position due to the Earth’s orbiting of the Sun.
Greeks didn’t detect parallax !
The nearest stars are much farther away than the Greeks thought.
The parallax angles of the stars are so small, that you need a telescope to observe them.
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Two Possible reasons why stellar parallax was not detected:
1. Stars are so far away that stellar parallax is too small for naked eye to notice.
2. Earth does not orbit Sun; it is the center of the universe.
Debate about theory: Earth-centered vs. Sun-centered Planetary System.
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Section
3 Scientific Theory: What is a good “Theory” ?
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Scientific Thinking
• It is a natural part of human curiosity: Search for understanding and truths that explain many facts.
• We draw conclusions based on our experiences.
• Progress is made through “trial and error.” Hypothesize. Then test your hypothesis.
Eating pasta makes me get fat . . .
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Nicolaus Copernicus
(1473-1543) He thought Ptolemy’s model was contrived.
Artificial construction; Too complicated.
De Revolutionibus Orbium Coelestium
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Copernicus’
Heliocentric
Model
• Sun is at center • Earth orbits like any other planet • Inferior planet orbits are smaller • Retrograde motion occurs when we “lap” Mars and the other superior planets © 2005 Pearson Education Inc., publishing as Addison-Wesley
Johannes Kepler
(1571-1630) • Greatest theorist of his day • Imagined planets on “heavenly spheres” © 2005 Pearson Education Inc., publishing as Addison-Wesley
Kepler’s Laws
1. Each planet’s orbit around the Sun is an
ellipse,
with the Sun at one focus.
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Eccentricity of an Ellipse © 2005 Pearson Education Inc., publishing as Addison-Wesley
Kepler’s 2nd Law A planet moves along its orbit with a speed that changes in such a way that a line from the planet to the Sun sweeps out equal areas in equal intervals of time.
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Kepler’s 3rd Law
The cube of a planet’s average distance from the Sun is equal to the square of its orbital period. (Use units of years and AUs.) a 3 = P 2 © 2005 Pearson Education Inc., publishing as Addison-Wesley
Curiosity Landing on Mars 5 August 2012